TEE(2) Linux Programmer's Manual TEE(2)
NAME
tee - duplicating pipe content
SYNOPSIS
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <fcntl.h>
ssize_t tee(int fd_in, int fd_out, size_t len, unsigned int flags);
DESCRIPTION
tee() duplicates up to len bytes of data from the pipe referred to by
the file descriptor fd_in to the pipe referred to by the file descrip-
tor fd_out. It does not consume the data that is duplicated from
fd_in; therefore, that data can be copied by a subsequent splice(2).
flags is a bit mask that is composed by ORing together zero or more of
the following values:
SPLICE_F_MOVE Currently has no effect for tee(); see splice(2).
SPLICE_F_NONBLOCK Do not block on I/O; see splice(2) for further de-
tails.
SPLICE_F_MORE Currently has no effect for tee(), but may be imple-
mented in the future; see splice(2).
SPLICE_F_GIFT Unused for tee(); see vmsplice(2).
RETURN VALUE
Upon successful completion, tee() returns the number of bytes that were
duplicated between the input and output. A return value of 0 means
that there was no data to transfer, and it would not make sense to
block, because there are no writers connected to the write end of the
pipe referred to by fd_in.
On error, tee() returns -1 and errno is set to indicate the error.
ERRORS
EAGAIN SPLICE_F_NONBLOCK was specified in flags or one of the file de-
scriptors had been marked as nonblocking (O_NONBLOCK), and the
operation would block.
EINVAL fd_in or fd_out does not refer to a pipe; or fd_in and fd_out
refer to the same pipe.
ENOMEM Out of memory.
VERSIONS
The tee() system call first appeared in Linux 2.6.17; library support
was added to glibc in version 2.5.
CONFORMING TO
This system call is Linux-specific.
NOTES
Conceptually, tee() copies the data between the two pipes. In reality
no real data copying takes place though: under the covers, tee() as-
signs data to the output by merely grabbing a reference to the input.
EXAMPLES
The example below implements a basic tee(1) program using the tee()
system call. Here is an example of its use:
$ date |./a.out out.log | cat
Tue Oct 28 10:06:00 CET 2014
$ cat out.log
Tue Oct 28 10:06:00 CET 2014
Program source
#define _GNU_SOURCE
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <limits.h>
int
main(int argc, char *argv[])
{
int fd;
int len, slen;
if (argc != 2) {
fprintf(stderr, "Usage: %s <file>\n", argv[0]);
exit(EXIT_FAILURE);
}
fd = open(argv[1], O_WRONLY | O_CREAT | O_TRUNC, 0644);
if (fd == -1) {
perror("open");
exit(EXIT_FAILURE);
}
do {
/*
* tee stdin to stdout.
*/
len = tee(STDIN_FILENO, STDOUT_FILENO,
INT_MAX, SPLICE_F_NONBLOCK);
if (len < 0) {
if (errno == EAGAIN)
continue;
perror("tee");
exit(EXIT_FAILURE);
} else
if (len == 0)
break;
/*
* Consume stdin by splicing it to a file.
*/
while (len > 0) {
slen = splice(STDIN_FILENO, NULL, fd, NULL,
len, SPLICE_F_MOVE);
if (slen < 0) {
perror("splice");
break;
}
len -= slen;
}
} while (1);
close(fd);
exit(EXIT_SUCCESS);
}
SEE ALSO
splice(2), vmsplice(2), pipe(7)
COLOPHON
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latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2020-06-09 TEE(2)